The present invention pertains to improvements in capacitance-variation-sensitive touch sensing array systems.
A touch sensing array system may include an array of individual touch responsive areas which may be operated as keys on a keyboard or may include a single relatively large touch responsive area which may be operated for control purposes as a touch pad to provide an indication associated with a given position in the array. In a capacitance-variation-sensitive touch sensing array system, the touching of a touch responsive area results in a variation of the capacitance between one or more electrodes and circuit ground. This variation results in the variation of a sense signal produced on one or more sense lines coupled to the respective electrodes. In a typical capacitance-variation-sensitive touch sensing array system, each electrode is driven by an alternating current or pulse signal. Sense signals are produced on the respective sense lines coupled to the respective driven electrodes. The sense signals are processed to determine whether a touch responsive area has been touched by an operator.
A typical touch sensing array has three types of conductors: drive lines, sense lines, and electrodes.
A drive line is a conductor coupled to a drive signal generator and capacitively coupled to one or more electrodes. Typically an enlarged portion of the drive line constitutes a plate of a drive line-to-electrode capacitor.
A sense line is a conductor coupled to a sense signal detector and capacitively coupled to one or more electrodes. Typically an enlarged portion of the sense line constitutes a plate of an electrode-to-sense line capacitor.
An electrode is a conductor capacitively coupled to at least one drive line and capacitively coupled to at least one sense line. Typically an electrode has one or more enlarged portions or plates, hereafter called tabs, which furnish a capacitor plate to complete a capactive coupling between the electrode and the body of an operator, and thereby to system ground, when the tab or the area overlying the tab is touched by a finger of the operator. The electrode part of a drive line-to-electrode capacitor or electrode-to-sense line capacitor may be one or more tabs of the electrode, or may be a conductive plate of the electrode not serving as a tab.
U.S. Pat. No. 4,145,748 to Eichelberger and Butler is relevant to the present invention. This patent describes a touch sensing array involving drive lines, sense lines, and electrodes. In this system, there is a unique electrode at the intersection of each drive line and sense line, and each electrode is associated with only one key. The number of keys is equal to the product of the number of drive lines and the number of sense lines. Under the direction of control logic means, voltage pulses are applied sequentially to the individual drive lines and the resulting signals on the sense lines are individually processed by an A/D converter to produce a digital output count uniquely associated with a pair made up of a single drive line and a single sense line and therefore with the unique electrode which lies at the intersection of said single drive line and said single sense line. The digital output associated with each electrode is compared to a "no-touch" digital count unique to that electrode which is stored in a memory and updated periodically by a count of at most one. Each comparison of a digital count associated with a specific electrode to the "no-touch" digital count associated with that electrode, results in an independent determination of the presence or absence of touch of that electrode. In the event of an extended dwell time of operator touch, the "no-touch" digital count becomes incorrect as an indicator of the absence of touch for the key(s) touched. This is because the periodic updates occur regularly, even during the dwell period. This condition may result in "false negatives" wherein the touch array is not responsive to operator touch of affected keys. The condition is corrected after a sufficient number of periodic updates in the absence of operator touch.
Other relevant teachings in the prior art are described below.
Several touch sensing array systems are described in U.S. Pat. No. 4,233,522 to Grummer and Hendriks. Each of these systems involves one drive line, a plurality of sense lines and a plurality of electrodes. The sense lines are divided into row sense lines and column sense lines and there are a number of keys equal to the product of the number of row sense lines and the number of column sense lines. Each sense line is connected to a unique detection circuit. A common feature of these systems is an array of touch actuated keys. Each key is composed in part of two tabs; each tab is a directly touchable conductive plate. The tabs are arranged on the top side of a substrate in a coordinate grid of rows and columns where each row of tabs is associated with a unique row sense line and each column of tabs is associated with a unique column sense line. Each key has one row tab and one column tab. Actuation of a selected key is brought about by concurrent direct touching of the two tabs of the key. In a first and second embodiment, the tabs of each row and the tabs of each column are electrically connected to form electrodes. This requires crossing over of selected conductor runs connecting the tabs. In the first embodiment, on the bottom surface of the substrate, there is a single drive line which delivers a common drive signal to each of the electrodes; the coupling of the drive line to the electrode is capacitive and is effected by a plurality of expanded conductive plates linked to the drive line and underlying a portion of each of the tabs of each electrode; also on the bottom surface of the substrate are a plurality of sense lines; there is a unique sense line for each row electrode and for each column electrode; each sense line is capacitively coupled to a particular row or column electrode by expanded conductive plates linked to the sense line and underlying the tabs of the electrode. Crossing over of selected conductor runs is required on the bottom surface of the substrate. In the second embodiment, there are no conductors on the bottom side of the substrate and the capacitive couplings of the drive line and of the sense lines to row and column electrodes are carried out using discrete components in an electrical circuit located separate from the touch panel. A third embodiment resembles the second embodiment except that there are two layers of conductive traces on the top side of the supporting substrate and crossing over of conductive runs is not required; these two layers of conductive traces are separated by a thin dielectric film; the tabs of the columns lie on the top side of this dielectric film and are connected by conductive traces as in embodiments one and two; the tabs of each row are also on the top side of the dielectric film but are not linked to form a single conductor; each tab in a row overlies an expanded portion of a single conductor on the bottom side of the dielectric film and thereby is strongly capacitively coupled to this single conductor which is coupled to the common drive line and to a unique sense line as in embodiment two. In all three of the embodiments, the detection circuits connected to the sense lines are scanned in a sequential manner to detect touch. Because of the direct touch of the touchable plates, a capacitive coupling to ground on the order of 100 pf is brought about by operator touch. Changes of this large magnitude can be detected by very simple means. Apart from the scanning process, each detection circuit operates independently to report a state of touch or no touch for a particular row or column. The logic of key touch reporting is based on the outputs of the independent detection circuits.
In U.S. Pat. No. 4,290,052 to Eichelberger et al., a physical layout of a touch sensing array is given. Touch sensing circuitry is fabricated on a double sided printed circuit board adhesively bonded to the bottom surface of an insulative substrate such as glass. Squarish touch electrodes are formed on the top side of the printed circuit board and transmitter (drive line) and receiver (sense line) electrode tabs in registration therewith are formed on the bottom side of the printed circuit board. Electrical components and interconnection leads are fabricated as an integral part of the conductor patterns on the surfaces of the printed circuit board. The number of keys is equal to the product of the number of drive lines and the number of sense lines.
In another type of touch sensing array system, the drive lines and sense lines are combined to form a plurality of drive/sense lines. A touch sensing array of this type is described in U.S. Pat. No. 3,757,322 to Harold Barkan et al. In this device, there is a coordinate grid made up of row and column electrodes wherein each electrode is coupled to a unique drive/sense line; there is a key at the intersection of each row and column electrode; and the number of keys is equal to the product of the number of row electrodes and the number of column electrodes. The operator concurrently and directly touches a tab of a row electrode and a tab of a column electrode to activate a key. A further touch sensing array having combined drive/sense lines is described in U.S. Pat. No. 4,288,786 to Ferenc Ledniczki. In this device, there are n pairs of drive/sense lines and 2 to the nth power electrodes; each electrode is capacitively coupled to one line in each pair of drive/sense lines; and each electrode is associated with a unique key.
Other types of keyboards share with touch sensing array systems the features of drive lines, sense lines, and sense signal processing. In particular, numerous capacitance actuated keyboards based on changes in capacitance brought about by mechanical displacement exist in the prior art and are in widespread use. Such keyboards are disclosed, for example, in U.S. Pat. Nos. 3,921,166 to Volpe, 4,305,135 to Dahl et al., and 4,359,720 to Chai et al.
Prior art capacitance-variation-sensitive touch sensing array systems are further described in U.S. Pat. Nos. 4,290,052 to Eichelberger et al; 4,379,287 to Tyler et al; 4,305,135 to Dahl et al; 4,157,539 to Hunts et al; 3,691,555 to Looschen; 4,321,479 to Ledniczki et al; 4,103,252 to Bobick; and 3,921,167 to Fox, and in Publication No. T904,008 by Crouse. Capacitance-variation tabs also are described in U.S. Pat. No. 3,492,440 to Cerbone et al and in IBM Technical Disclosure Bulletin, Vol. 17, No. 1, June 1974, page 166-7, "Touch-Sensing Circuit", by J. A. Williams.